geology of seattle - university of washington
TRANSCRIPT
Geology of SeattleJohn Dawson Stephen Guarente
repeated glaciations for the past 2.4m.y.
• The last glacier to override the area was the Vashon glacier (reached @ ~14,500 14C yr B.P & retreated by ~13,650 14C yr B.P). as part of the Fraser Glaciation ( 30-10 ka)
• Pre-Fraser, was the Olympia glaciation (65- 15ka)
Fraser Glaciation advancement and result
Plate Tectonics & Faults
• Cascadia subduction zone
What and where is the Seattle Fault Zone?
Downtown Seattle Geological Map
Maps: Simplified Geology & Grain Sizes
Importance of Glacial Deposits
• Important for conducting drilling
Viaduct Map
Fig 1.12
Fig 1.13
Seismicity and the Alaskan Way Viaduct
• Viaduct built from 1949 through 1953 • Has sustained 2 earthquakes of Mw
> 6.5 since 1965 • 1st EQ was in 1965: Mw=6.5;
epicenter only 15 miles from Viaduct • 2nd EQ was known as the Nisqually
EQ in 2001
Nisqually Earthquake
•Date & Time of Occurrence: Wednesday •February 28, 2001 at 10:54:32.78 AM(PST) •Magnitude: Mw=6.8 •Moment Magnitude:
[M0 is the seismic moment in dyne centimetres (10-7 Nm)]
•Depth: 52.40 km •Location: 47.1525N 122.7197W
ShakeMap [Mercalli Scale]
Liquefaction Susceptibility
Liquefaction Susceptibility of Downtown Seattle
Liquefaction Hazard Definition: softening and/or weakening of a soil layer due to
the generation of excess pore water pressure. (u↑, σ’↓)
Susceptibility Criteria:
• Geologic ~ Hydraulic fill and tide flat deposits are present for approximately the first 50 feet below the existing viaduct.
• Compositional ~ Loose sand below the water table.
— Uniform graded soils with rounded particles most susceptible.
Alignment Cross-Section
– Viaduct settled 5+ inches.
• A function of porewater pressure dissipation and particle rearrangement upon termination of ground motions.
• Increased lateral load weakened seawall structure.
Mechanisms: • Flow liquefaction: static shear stress in soil deposit is greater than
the steady-state residual strength of the soil. Loose sands only.
• Cyclic mobility: cyclic shear stresses from ground motions induce excess porewater pressure causing the steady-state strength to be exceeded momentarily. This results in incremental deformation known as lateral spreading.
Consequences of future liquefaction event:
• Lateral spreading would significantly increase lateral load on already weakened seawall.
• Further settlement would cause increased stress on and deformation of the Viaduct structure and foundation.
• Failure of local sections of the structure at a minimum.
Seawall replacement:
highway. • Single and double
column steel-reinforced concrete structure
Case Study: Cypress Street Viaduct
• Damaged spectacularly in the 1989 Loma Prieta earthquake (6.9 MMS).
Case Study: Cypress Street Viaduct
• 42 people were killed, and many more were injured as portions of the upper roadway fell onto the lower roadway, crushing the vehicles there.
Case Study: Cypress Street Viaduct
Case Study: Cypress Street Viaduct
• Built on fill, and bay mud, the structure was subjected to amplified ground movements during the earthquake.
• The primary cause of failure was the lack of steel reinforcing connections between the upper and lower column segments.
Case Study: Cypress Street Viaduct
• A quote from the mayor’s board of inquiry:
“The reinforcement in the columns and girders was generally poorly detailed by current standards and reflects the engineering profession’s lack of understanding regarding the inelastic response of reinforced concrete members at the time when these structures were designed. … A lack of redundancy and the inadequate reinforcement detailing are two of the major seismic deficiencies in these freeway structures.”
• Essentially saying: People didn’t know how to build earthquake-resistant structures back then.
Building a Liquefaction Resistant Alaskan-Way Viaduct
• Three Aspects to Consider for Construction:
• Avoid soils susceptible to liquefaction. (not possible in this case)
• Improve the soil being tunneled into or built upon. (i.e. drainage options to eliminate pore pressure within soil)
• Build a liquefaction resistant Structure.
– Improve ductility to allow for potentially large deformations via seismic activity and failure due to liquefaction.
– Account for vertical and horizontal loads and moments.
– Design a drainage system into the structure to prevent liquefaction within the soil.
• Improving the Ductility of Structure or Subsurface Tunnel.
• Increase ductility of the structure by connecting the structure within the liquefiable layer to a stiffer layer via reinforced pile foundations.
– Allows superstructure to move as a whole when liquefaction causes failure within the soil during seismic events.
• Improve ductility within the connection of the tunnel to the pile foundations via seismic bearings or similar ductile connection.
http://www.ce.washington.edu/%7Eliquefaction/html/how/ resistantstructures.html
• Accounting for Moments, Vertical loads and Horizontal deformation due to Liquefaction:
– Moment within the Viaduct Structure due to liquefaction can be significantly reduced via ductile connections to the pile foundations (i.e. Seismic Bearings if construction is above the surface).
– Horizontal loads can be reduced with the use of large reinforced pile foundations connected to a stiff foundation to allow for lateral deformation without failure.
– Vertical compression loads due to liquefaction will be accounted for within the structural design of the reinforced concrete piles extending to the stiff foundation. Vertical tension loads will not be accounted for unless expansive clays exist between the liquefiable layer and the stiff foundation below surface, or under the tunnel structure.
Examples of Bored Tunnels
• Due for completion later this year
• Diameter 15 meters • 8.5 km • Why use TBM? • Similarities to Alaskan
Way
Shield System • Similarities to Alaskan Way
London Jubilee Line Extension • $375 m/km
• 1.4 billion GBP over budget (67%)
• 2 years late (40%)
• “Time and cost overruns could have been minimized with a more established strategy at the beginning of the project.” Arup Report
(Reference: T&T, October 2000, p19)
City of Miami Port Tunnel
• Deal collapsed December 2008 • "Nobody anticipated this kind of increase.
This is just a reality of doing business in 2008. You just have to get used to it." MTA Board Chairman Dale Hemmerdinger ENR, January 31, 2008
Boston Big Dig
• 3.5 mile (5.6 km) tunnel under the city
• Had to find safest way to build the tunnel without endangering existing highway above
• $14.6 billion (143%) • “wouldn’t it be cheaper
to raise the city” • Lessons learnt...
Channel Tunnel
deep • The cost overran
from both the UK and France cut through chalk marl
• Geology of the Channel
Cut and cover tunnel alternative:
• Steps: 1. Soil excavated. 2. Tunnel constructed. 3. Soil placed on top.
• Challenges:
• Tunnel would have to resist huge lateral earth pressures from future seismic event and downward flowing groundwater. West tunnel wall would serve as seawall replacement.
• Dewatering of the excavation would be a massive undertaking given the high groundwater table and proximity to Puget Sound.
• Cracking of structure must be closely monitored during design-life to protect against saltwater inflow and subsequent corrosion.
• Cracking may cause water inflow and potential flooding.
• Utility relocation extensive due to location.
Bored tunnel alternative:
• Calls for tunnel to be constructed within dense glacial till, well below liquefiable soils and away from seawall.
• Liquefiable soil present at south approach.
– Deep-soil mixing with jet grout would increase the density and strength while greatly reducing the permeability of the soil. This is very expensive but effective proven effective. (Big Dig)
• Slurry walls at south approach would extend downwards to 100 feet or more below grade, thus subjecting them to significant lateral loads due to earth and hydrostatic pressure.
• Significant excavation required at north approach to reconnect with surface roads. This will require extensive excavation support and water retention system.
Two-lane stacked bored tunnel
• Structure would be susceptible to and must resist liquefaction hazard.
– Significant design challenges.
• Aesthetic and commercial issues.
deformations.
References • Liquefaction, University of Washington
http://www.ce.washington.edu/%7Eliquefaction/ • PNSN: Pacific Northwest Seismic Network,
University of Washington http://www.pnsn.org/SEIS/EQ_Special/WEBDIR_01022818543p/welcome.html • USGS: United States Geological Survey
Plate Tectonics & Faults
Downtown Seattle Geological Map
Maps: Simplified Geology & Grain Sizes
Importance of Glacial Deposits
Waterfront Map, Boring, Cross-Section
Nisqually Earthquake
Liquefaction Susceptibility
Liquefaction Hazard
Building a Liquefaction Resistant Alaskan-Way Viaduct
Slide Number 34
Slide Number 35
Slide Number 36
Boston Big Dig
repeated glaciations for the past 2.4m.y.
• The last glacier to override the area was the Vashon glacier (reached @ ~14,500 14C yr B.P & retreated by ~13,650 14C yr B.P). as part of the Fraser Glaciation ( 30-10 ka)
• Pre-Fraser, was the Olympia glaciation (65- 15ka)
Fraser Glaciation advancement and result
Plate Tectonics & Faults
• Cascadia subduction zone
What and where is the Seattle Fault Zone?
Downtown Seattle Geological Map
Maps: Simplified Geology & Grain Sizes
Importance of Glacial Deposits
• Important for conducting drilling
Viaduct Map
Fig 1.12
Fig 1.13
Seismicity and the Alaskan Way Viaduct
• Viaduct built from 1949 through 1953 • Has sustained 2 earthquakes of Mw
> 6.5 since 1965 • 1st EQ was in 1965: Mw=6.5;
epicenter only 15 miles from Viaduct • 2nd EQ was known as the Nisqually
EQ in 2001
Nisqually Earthquake
•Date & Time of Occurrence: Wednesday •February 28, 2001 at 10:54:32.78 AM(PST) •Magnitude: Mw=6.8 •Moment Magnitude:
[M0 is the seismic moment in dyne centimetres (10-7 Nm)]
•Depth: 52.40 km •Location: 47.1525N 122.7197W
ShakeMap [Mercalli Scale]
Liquefaction Susceptibility
Liquefaction Susceptibility of Downtown Seattle
Liquefaction Hazard Definition: softening and/or weakening of a soil layer due to
the generation of excess pore water pressure. (u↑, σ’↓)
Susceptibility Criteria:
• Geologic ~ Hydraulic fill and tide flat deposits are present for approximately the first 50 feet below the existing viaduct.
• Compositional ~ Loose sand below the water table.
— Uniform graded soils with rounded particles most susceptible.
Alignment Cross-Section
– Viaduct settled 5+ inches.
• A function of porewater pressure dissipation and particle rearrangement upon termination of ground motions.
• Increased lateral load weakened seawall structure.
Mechanisms: • Flow liquefaction: static shear stress in soil deposit is greater than
the steady-state residual strength of the soil. Loose sands only.
• Cyclic mobility: cyclic shear stresses from ground motions induce excess porewater pressure causing the steady-state strength to be exceeded momentarily. This results in incremental deformation known as lateral spreading.
Consequences of future liquefaction event:
• Lateral spreading would significantly increase lateral load on already weakened seawall.
• Further settlement would cause increased stress on and deformation of the Viaduct structure and foundation.
• Failure of local sections of the structure at a minimum.
Seawall replacement:
highway. • Single and double
column steel-reinforced concrete structure
Case Study: Cypress Street Viaduct
• Damaged spectacularly in the 1989 Loma Prieta earthquake (6.9 MMS).
Case Study: Cypress Street Viaduct
• 42 people were killed, and many more were injured as portions of the upper roadway fell onto the lower roadway, crushing the vehicles there.
Case Study: Cypress Street Viaduct
Case Study: Cypress Street Viaduct
• Built on fill, and bay mud, the structure was subjected to amplified ground movements during the earthquake.
• The primary cause of failure was the lack of steel reinforcing connections between the upper and lower column segments.
Case Study: Cypress Street Viaduct
• A quote from the mayor’s board of inquiry:
“The reinforcement in the columns and girders was generally poorly detailed by current standards and reflects the engineering profession’s lack of understanding regarding the inelastic response of reinforced concrete members at the time when these structures were designed. … A lack of redundancy and the inadequate reinforcement detailing are two of the major seismic deficiencies in these freeway structures.”
• Essentially saying: People didn’t know how to build earthquake-resistant structures back then.
Building a Liquefaction Resistant Alaskan-Way Viaduct
• Three Aspects to Consider for Construction:
• Avoid soils susceptible to liquefaction. (not possible in this case)
• Improve the soil being tunneled into or built upon. (i.e. drainage options to eliminate pore pressure within soil)
• Build a liquefaction resistant Structure.
– Improve ductility to allow for potentially large deformations via seismic activity and failure due to liquefaction.
– Account for vertical and horizontal loads and moments.
– Design a drainage system into the structure to prevent liquefaction within the soil.
• Improving the Ductility of Structure or Subsurface Tunnel.
• Increase ductility of the structure by connecting the structure within the liquefiable layer to a stiffer layer via reinforced pile foundations.
– Allows superstructure to move as a whole when liquefaction causes failure within the soil during seismic events.
• Improve ductility within the connection of the tunnel to the pile foundations via seismic bearings or similar ductile connection.
http://www.ce.washington.edu/%7Eliquefaction/html/how/ resistantstructures.html
• Accounting for Moments, Vertical loads and Horizontal deformation due to Liquefaction:
– Moment within the Viaduct Structure due to liquefaction can be significantly reduced via ductile connections to the pile foundations (i.e. Seismic Bearings if construction is above the surface).
– Horizontal loads can be reduced with the use of large reinforced pile foundations connected to a stiff foundation to allow for lateral deformation without failure.
– Vertical compression loads due to liquefaction will be accounted for within the structural design of the reinforced concrete piles extending to the stiff foundation. Vertical tension loads will not be accounted for unless expansive clays exist between the liquefiable layer and the stiff foundation below surface, or under the tunnel structure.
Examples of Bored Tunnels
• Due for completion later this year
• Diameter 15 meters • 8.5 km • Why use TBM? • Similarities to Alaskan
Way
Shield System • Similarities to Alaskan Way
London Jubilee Line Extension • $375 m/km
• 1.4 billion GBP over budget (67%)
• 2 years late (40%)
• “Time and cost overruns could have been minimized with a more established strategy at the beginning of the project.” Arup Report
(Reference: T&T, October 2000, p19)
City of Miami Port Tunnel
• Deal collapsed December 2008 • "Nobody anticipated this kind of increase.
This is just a reality of doing business in 2008. You just have to get used to it." MTA Board Chairman Dale Hemmerdinger ENR, January 31, 2008
Boston Big Dig
• 3.5 mile (5.6 km) tunnel under the city
• Had to find safest way to build the tunnel without endangering existing highway above
• $14.6 billion (143%) • “wouldn’t it be cheaper
to raise the city” • Lessons learnt...
Channel Tunnel
deep • The cost overran
from both the UK and France cut through chalk marl
• Geology of the Channel
Cut and cover tunnel alternative:
• Steps: 1. Soil excavated. 2. Tunnel constructed. 3. Soil placed on top.
• Challenges:
• Tunnel would have to resist huge lateral earth pressures from future seismic event and downward flowing groundwater. West tunnel wall would serve as seawall replacement.
• Dewatering of the excavation would be a massive undertaking given the high groundwater table and proximity to Puget Sound.
• Cracking of structure must be closely monitored during design-life to protect against saltwater inflow and subsequent corrosion.
• Cracking may cause water inflow and potential flooding.
• Utility relocation extensive due to location.
Bored tunnel alternative:
• Calls for tunnel to be constructed within dense glacial till, well below liquefiable soils and away from seawall.
• Liquefiable soil present at south approach.
– Deep-soil mixing with jet grout would increase the density and strength while greatly reducing the permeability of the soil. This is very expensive but effective proven effective. (Big Dig)
• Slurry walls at south approach would extend downwards to 100 feet or more below grade, thus subjecting them to significant lateral loads due to earth and hydrostatic pressure.
• Significant excavation required at north approach to reconnect with surface roads. This will require extensive excavation support and water retention system.
Two-lane stacked bored tunnel
• Structure would be susceptible to and must resist liquefaction hazard.
– Significant design challenges.
• Aesthetic and commercial issues.
deformations.
References • Liquefaction, University of Washington
http://www.ce.washington.edu/%7Eliquefaction/ • PNSN: Pacific Northwest Seismic Network,
University of Washington http://www.pnsn.org/SEIS/EQ_Special/WEBDIR_01022818543p/welcome.html • USGS: United States Geological Survey
Plate Tectonics & Faults
Downtown Seattle Geological Map
Maps: Simplified Geology & Grain Sizes
Importance of Glacial Deposits
Waterfront Map, Boring, Cross-Section
Nisqually Earthquake
Liquefaction Susceptibility
Liquefaction Hazard
Building a Liquefaction Resistant Alaskan-Way Viaduct
Slide Number 34
Slide Number 35
Slide Number 36
Boston Big Dig